To meet industry needs for electrical power converters which convert AC to DC without injecting large amounts of harmonic currents into the power system, numerous transformer and converter topologies are available. A 15-phase transformer supplying a 30-pulse bridge rectifier provides means to ensure that an almost pure sine wave of current is drawn from a three-phase power supply. It ensures compliance with the most exacting harmonic current specifications.
There are few, if any, descriptions of 30-pulse converters in the public domain. However, some of the unique design issues relating to autotransformers for multi-pulse converters are described in U.S. Pat. No. 7,274,280. With appropriate design no interphase reactors are required. Using unique winding interconnections on the associated phase shifting transformer, means are provided to cater for third harmonic currents in the transformer output leads, such currents being caused by the inherent load current demands of many converter circuits.
Some autotransformer designs accommodate third harmonic current by means of a separate delta winding. This winding adds to the autotransformer power rating.
In other autotransformer designs, it may be feasible to eliminate the delta winding by taking advantage of the zero-sequence impedance associated with a three-phase core form structure. This impedance functions as a “hidden delta”, as described in the paper by J. E. Armes, et. al., but external fluxes caused by the “hidden delta” may be the source of unwanted eddy current losses in the structures surrounding the transformer. Also, the performance is affected by parameters normally outside the control of the transformer designer.
The invention described here ensures a predictable path for third harmonic and other zero-sequence currents by appropriate selection and routing of the autotransformer windings. It does not require a separate delta winding and uses nine windings per phase. The invention can be used as the secondary configuration of a double-wound transformer with any three-phase input winding topology.
A variation of the invention can be used as the secondary arrangement of a double-wound transformer that includes a delta circuit in the primary. The same variation is feasible as an autotransformer when the “hidden delta” effect is tolerable, or other non-isolated or isolated delta circuits are connected.